In the magnetic recording media such as used for magnetic discs, development is now under progress for making the recording density higher and it is known that the value for the magnetization transition width (.mu.m) a represented by the following equation (1) has to be decreased in order to improve the recording density. EQU a.varies..delta..multidot.Br/(m.multidot.Hc) (1)
where .delta. represents the thickness (.mu.m) of a magnetic layer, Br represents the residual magnetic flux density (G), m represents a factor regarding the squareness and Hc represents the coercive force (Oe).
Accordingly, for improving the magnetic density, it is an effective means for improving the coercive force together with the reduction of the thickness for the magnetic layer.
By the way, the magnetic recording media are generally classified into three types, i.e., (1) a coating type, (2) a plated thin film type and (3) a sputtered thin film type depending on the difference of the manufacturing method, and improvement for the coercive force has been tried to each of the cases as shown below.
The coating type medium (1) is a magnetic recording medium prepared by coating and baking acicular .gamma.-Fe.sub.2 O.sub.3 magnetic particles mixed with a binder and the like on an aluminum alloy substrate and, for the improvement of the coercive force, it has been employed a method, for example, of refining the acicular magnetic particles or depositing Co to the surface thereof.
Further, the plated thin film type medium (2) is prepared by forming a magnetic layer such as Co-P or Co-Ni-P by means of an electroless plating method on a substrate comprising an aluminum alloy substrate (hereinafter referred to as NiP plated substrate), in which improvement is intended for the coercive force by the improvement of the plating bath composition.
However, it is difficult to reduce the film thickness in the coating type medium (1), while no satisfactory increase in the recording density has yet been attained in the plated thin film type medium (2) above and, afterall, expectation has been made to the method of forming the magnetic layer by means of the sputtering method (3) above.
As the sputtering type thin film medium, those prepared by forming a magnetic layer such as Co-Ni-Cr or Co-Ni by means of sputtering on a NiP plated substrate is ordinary, and improvement for the magnetic composition has been attempted as a means for improving the coercive force. Further, there have also been proposed a method of forming a film of a magnetic layer in a state where the temperature of the substrate is elevated (for example, in the Summary of The Academic Lecture of the Eleventh Meeting of The Applied Magnetic Society of Japan by Ishikawa, et al, p 18, 1987, 11), and a method of optimizing the conditions for forming the magnetic layer by applying a reverse bias voltage to a substrate (for example, in the Preprint for the Associated Lecture of 35th Applied Physics Society, by Hashimoto et al, p 57, 1988, 10).
However, even in the sputtering thin film type medium as described above, since a noble metal such as Co-Cr-Pt is used as the magnetic material as the method relying on the improvement of the magnetic composition, it is not economically advantageous.
Further, in the case of high temperature film-forming method of forming a magnetic layer by means of sputtering in a state of elevating the temperature of the substrate, although a magnetic recording medium improved with the coercive force can be obtained in the experimental level, if it is intended for mass production, the magnetic layer is not formed easily in a state where the temperature of heating the substrates exceeds 250.degree. C. due to the problem in view of the film forming device such as the carrier for holding the substrate is liable to be deformed by heating. In addition, in the case of using the NiP plated substrate, there has been a problem that on amorphous NiP plated layer is crystallized and magnetized to give an undersired effect on the magnetic layer at higher than 280.degree. C. and, further, deformation is caused to the substrate if it is heated to higher than 300.degree. C.
On the other hand, the method of forming the magnetic layer in a state where a reversed bias voltage is applied to the substrate provides an excellent effect for the improvement of the coercive force but it involves a problem that the structure of the film forming device is complicated due to the requirement for applying the reverse bias voltage. In this way, even the sputtering thin film type medium is still insufficient for the increase of the recording density.
By the way, as the performance required for the substrate in view of attaining the increase of the recording density, there can be mentioned, for example, that it has a surface property with less surface roughness and no surface defects, it is chemically stable as a underlying base for the magnetic recording layer and it has such a hardness and strength as capable of ensuring durability against contact with a head. Further, as the performance required for the material of the substrate, there can be mentioned non-magnetic property, high hardness, high heat resistance, reduced weight, high strength and high rigidity.
For satisfying such requirements, magnetic recording medium using, as a substrate, a vitreous carbon substrate has recently been proposed. For instance, Japanese Patent Laid-Open Sho 62-234232 discloses a magnetic disc in which a magnetic thin film is formed on a vitreous carbon substrate and, further, the present inventors have also developed and have already filed an application (Japanese Patent Application Hei 1-188225) for a magnetic thin recording medium in which a thin Co-based alloy film is formed on a vitreous carbon substrate. Although these magnetic recording media can be evaluated as having high magnetic recording reliability, they can not yet been said to be satisfactory in view of making the coercive force higher for the improvement of the recording density.